Method for producing deep drawing steel

ABSTRACT

POUR STEEL INGOT CONTAINING 0.06-0.12 WEIGHT PERCENT CARBON, 0.20-0.60 WEIGHT PERCENT MANGANESE, LESS THAN 0.03 PHOSPHOROUS, 0.05 MAXIMUM COPPER, 1.00 WEIGHT PERCENT MAXIMUM METALLOID, RESIDUAL SULFUR. INITIALLY ALLOW INGOT TO RIM, THEN CORE KILL WITH DEOXIDIZING AGENT TO PROVIDE HOMOGENOUS INGOT CORE. ROLL INGOT INTO STRIP. ANNEAL STRIP TO GIVE UNIFORM RECRYSTALLIZED GRAIN SIZE OF 7-8 ASTM (FERRITIC). DECARBURIZE TO 0.008 WEIGHT PERCENT MAXIMUM CARBON. COOL WITHOUT FURTHER ANNEAL. TEMPER ROLL. RESULTING PRODUCT IS READILY DEEP DRAWN AND READILY COATED WITH PORCELAIN ENAMEL.

United States Patent 3,556,866 METHOD FOR PROISJUCING DEEP DRAWING TEEL Melvin B. Gibbs, Harvey, 11]., and James L. Golding, Valparaiso, and Barry H. Levine, Highland, Ind., and John P. Novak, Homewood, IIL, assignors to Inland Steel Company, Chicago, 11]., a corporation of Delaware No Drawing. Filed Jan. 31, 1968, Ser. No. 701,866 Int. Cl. C21d 7/00, 7/14 US. Cl. 148-2 6 Claims ABSTRACT OF THE DISCLOSURE Pour steel ingot containing 0.06-0.12 weight percent carbon, 0.200.60 weight percent manganese, less than 0.03 phosphorous, 0.05 maximum copper, 1.00 weight percent maximum metalloid, residual sulfur. Initially allow ingot to rim, then core kill with deoxidizing agent to provide homogenous ingot core. Roll ingot into strip. Anneal strip to give uniform recrystallized grain size of 7-8 ASTM (ferritic). Decarburize to 0.008 weight percent maximum carbon. Cool without further anneal. Temper roll. Resulting product is readily deep drawn and readily coated with porcelain enamel.

BACKGROUND OF THE INVENTION The present invention relates to a method for producing a steel which may be subjected to a severe deep drawing operation and which then may be readily porcelain enameled. Deep drawing is an operation in which a sheet or strip of steel is deformed into a relatively intricate shape by the action of a press forcing the strip against a die.

A typical product of a severe deep drawing operation is a rectangular pan of relatively substantial depth, such as a vegetable pan in a refrigerator. It is important that the steel undergo such deformation without breaking or tearing during the drawing operation, and that the steel have a composition and be produced by a processing operation which impart the desired drawing properties to the steel without adverse effects on other properties of the steel and without requiring expensive or lengthy or otherwise undesirable processing conditions.

SUMMARY OF THE INVENTION In the method of the present invention the deep drawing steel is rolled from an ingot poured from molten steel having a carbon content between 0.06 and 0.12 weight percent, with essentially residual phosphorous and sulfur contents, and with low copper and other metalloid contents. A solid rim is initially allowed to form around the ingot, after which the molten core of the ingot is killed by injecting a deoxidizing agent, such as aluminum, down into the molten core of the ingot. The deoxidizing element is dispersed in the molten core and is injected in a quantity sufiicient to stop the evolution of rimming-causing gas, and this minimizes segregation in the ingot core. The ingot is then allowed to solidify and is rolled into a strip. As used herein, the term strip is generic to both strip and sheet.

The steel strip is then annealed at an elevated temperature, below the A temperature for the steel, to recrystallize the steel and to provide it with a ferritic grain size of about 7-8 on the ASTM scale. Because of the relatively high carbon content of the steel, the grain size is substantially uniform throughout the entire strip at the conclusion of the recrystallizing step, whereupon the strip is decarburized until the carbon content is no greater than 0.008 weight percent, following which the steel is cooled without further substantial annealing, at a rate which avoids quenching the steel. The steel is then temper rolled.

Patented Jan. 19, 1971 After the sequence of processing steps described above, the steel is readily susceptible to deep drawing and is also readily susceptible to being coated with only a single coat of porcelain enamel after the deep drawing operation.

Other features and advantages are inherent in the method claimed and disclosed or will become apparent to those skilled in the art from the following detailed description.

DESCRIPTION OF THE PREFERRED EMBODIMENTS The molten steel poured into the ingot should have a carbon content, in weight percent, of 0.06-0.12%. Below 0.06 weight percent, it is difficult to obtain a uniform grain size during annealing. This is because the lower the carbon content, the more difficult it is to obtain grain recrystallization upon annealing at a temperature below the A temperature for the steel; and the more difficult the recrystallization, the less uniform the grain size. It is important to have a uniform grain size because this reduces breakage or tearing of the steel during the deep drawing operation. The A, temperature of the steel is that temperature at which the matrix in the microstructure of the steel begins to change from ferrite to austenite upon heating.

The carbon content should not exceed 0.12% because a higher carbon content would adversely affect the rimming action. The best rimming action occurs at about 0.08-0.10 Weight percent carbon. Rimming is desirable because it improves the surface of the steel. Moreover, because the steel must eventually be decarburized, a car bon content no greater than 0.12 weight percent reduces the cost of decarburization compared to a higher carbon content.

The manganese content of the steel should be 0.20-0.60 weight percent. A manganese content lower than 0.20 weight percent may cause hot shortness difliculties during hot rolling and recrystallization difficulties during annealing. A manganese content substantiall above 0.60 weight percent would be undesirable in a product to be porcelain enameled because it would reduce the strength of the steel at the elevated temperature at which the porcelain enameled product is fired, causing the enameled product to sag or lose its shape.

The phosphorous content should be less than 0.03 weight percent. The low phosphorous content minimizes cold shortness during cold rolling and improves the ductility of the steel.

The sulfur content should be 0.03 weight percent, maximum. The low sulfur content minimizes hot shortness during hot rolling and improves the ductility of the steel.

The copper content should be 0.05 weight percent, maximum, with 0.03 weight percent preferred. The lower the copper content, the better the pickling rate for the steel. The pickling rate is the amount of material which can be removed from the steel in a pickling bath in a given period of time. A pickling operation occurs after the deep drawing operation and before the deep drawn product is porcelain enameled. The more material removed during the pickling bath, the more improved is the adherence of the porcelain enamel to the steel base of the product.

Improved enamel adherence characteristics for the steel base is especially important when the steel base is to be iven only a single coat of porcelain enamel (e.g., a white cover coat), a single coat being desirable over a double coat (generally a white cover coat over a blue ground coat) because the single coat costs less.

More specifically, for a two-coat enamel application (e.g., one coat of blue followed by a coat of white) only /2% grams of material per square foot of steel strip need be removed from the steel base for proper adherence of porcelain enamel; but, for a one-coat white porcelain enamel application, a removal of at least two grams per square foot is required. Using a pickling bath containing 9% H 80 at a temperature of 170 F., in min. about 4 grams per square foot can be removed with a copper content of 0.05 weight percent maximum.

Therefore, by maintaining the copper content low, the pickling characteristics of the material are improved, and a one-coat enamal operation may be utilized, thus improving the economics of the enameling operation. If it is desired to use two coats of enamel, the copper content may be a bit higher than 0.05 weight percent.

It is also important that the metalloid content of the steel be low to improve the pickling characteristics of the steel. Metalloids are elements such as nickel, arsenic, tin, etc. which cannot be removed from the steel by oxidation. The total of all metalloids should be 1.00 weight percent maximum. It is also important that the nickel and arsenic contents be relatively low because these elements discolor the base metal and adversely affect the adherence of the porcelain enamel on the end product, especially if the porcelain enamel is white.

In summary, the steel should consist essentially of, in

weight percent:

Carbon 0.06-0.12

Manganese 0.200.60 Phosphorous 0. 03 Sulfur (maximum) 0.03 Copper (maximum) 0.05 Metalloid (maximum) 1.00 Iron Balance A molten steel having the above composition is poured into an ingot mold, and the molten steel is allowed initially to rim in the ingot mold. A rimmed ingot gives a better surface on a finished rolled product than does an ingot which is not permitted to rim.

After the ingot has been allowed initially to rim, it is core killed by injecting a deoxidizing element downwardly into the interior of the still-molten core of the ingot. The deoxidizing element (e.g., one or more of aluminum, boron, vanadium, titanium or zirconium) is injected in a quantity sufficient to stop the evolution of rimmingcausing gas. For example, 0.02-0.08 weight percent aluminum would be a sufficient quantity for deoxidization, with 0.03 weight percent aluminum being a preferred amount. As an alternative, 01002-0003 weight percent boron or 0.02-0.05 weight percent vanadium, or various combinations of the various deoxidizing elements, may be used.

In a typical operation, core killing is performed by injecting aluminum shot into the molten core of the ingot at a time when the ingot has been poured almost to the top but before the conclusion of the pouring operation. Typically, the ingot runs from 60 to 9-0 inches high, the 90 inch ingot being a hot top ingot in which the top 15 inches of molten metal are poured into an insulated reservoir (hot top) for filling a shrinkage cavity which forms in the ingot portion below the hot top. A hot top ingot is the preferred embodiment, but an open top ingot can be used. The aluminum is generally added during the time the top 8 or 9 inches of the ingot is poured. The aluminum shot is gravity fed through a downwardly directed tubular gun barrel pointed at a stream of molten metal flowing downwardly from a ladle into the ingot mold. The descending stream of molten metal carries the aluminum down into the still-molten core of the ingot, and the aluminum is dispersed throughout the molten core.

Core killing the molten interior of the ingot prevents segregation of elements in the core, and this is especially true of elements such as carbon and sulfur. Preventing segregation in the ingot core provides more uniform properties throughout the steel rolled from the ingot. The

more homogenous structure resulting from core killing improves the ductility of the steel over the same steel without core killing, and thus provides better drawing properties.

By adding a core killing element such as aluminum, the aging properties of the steel are depressed compared to the same steel without aluminum. This is because the aluminum, or other deoxidizing element, ties up the nitrogen in the steel, thus providing a higher tolerance for total nitrogen than in the same steel without aluminum. For example, with an aluminum content of 0.02 weight percent, a total nitrogen content of 0.005 weight percent can be tolerated. With more aluminum, more nitrogen can be tolerated.

Using core killing, as defined above, the homogeneity of the core is better than that achieved by standard chemical capping practice, and the homogeneity is closer to that achieved in a fully killed steel.

After core killing, the ingot is allowed to solidify and is then rolled into strip of the desired thickness using conventional hot rolling and cold rolling procedures and an intermediate step of pickling.

After cold rolling, the strip is wound into an open coil, using conventional open coil winding techniques; and the open wound coil is placed within a conventional bell-type annealing furnace and heated in a non-oxidizing atmosphere until the temperature of the coil is about 1300 F. This is a temperature below the A for the steel. As the temperature of the steel is being raised to 1300, the steel is in the temperature range 1100-1300 F. for about two hours. During this time, recrystallization of the grains of the steel occurs; and a grain size (ferritic) of about 7-8 on the ASTM scale is obtained essentially uniformly throughout the entire steel. A uniform grain size is obtained because the steel has a carbon content between 0.06 and 0.12 weight percent.

When recrystallization has been completed, a decar-burizing atmosphere is circulated through the furnace. The decar burizing atmosphere may be steam or carbon dioxide. Steam is preferred. Decarburizing atmosphere is circulated through the annealing furnace until the carbon content of the steel is reduced to 0.008 weight percent maximum. Decarburizing is performed at a temperature at least as high as the recrystallizing temperature, but below the A (e.g., 1300 F.).

The carbon content of the steel is continuously monitored during the decar-burizing operation; and when the desired carbon con-tent is obtained, the decarburizing atmosphere is entirely eliminated from the interior of the furnace. At the conclusion of the decarburizing step, the grain size is still substantially the same as before the decarburizing step and the grain size is still substantially uniform throughout the steel strip.

At the conclusion of the decarburizing step, the atmosphere around the strip is rendered non-oxidizing; and, when this has been accomplished, heating of the furnace is stopped. When steam is the decarburizing agent, the interior of the furnace is dried out to a dew point at most, about 20 F. Immediately after the furnace interior has dried out, the strip is cooled, from the elevated temperature at which decarburization was performed, at a moderate cooling rate to avoid quenching the steel. Typically, the strip is allowed to remain in the furnace, itself undergoing cooling, until the strip temperature starts to drop. Then the furnace is removed from around the strip, and the strip is cooled under a protective cover which main tains a deoxidizing atmosphere around the strip as it cools.

Once the desired reduced carbon content has been obtained, there is no substantial further anneal of the steel. Cooling the steel essentially immediately after the conclusion of decarburizing, or as soon thereafter as is possible, avoids the possibility of increased grain size and of non-uniformity of grain size, both of which could occur with additional annealing after decar'burizing and both of which would have a negative effect on the drawa-bility of the steel.

Steel in accord- Same steel ance with present without core Property invention killing Yield strength, p.s.i 27, 70032, 650 28, 300-35, 400

Tensile strength, p.s i 44, 50046, 250 41, 700-44, 900

Percent elongation 42-45 35-43 Pickle rate (material removed in grams per sq. ft.) 6. 3-9. 7

value 1. 18-1. 35 1. 12 1 value (utter 200 days of aging) 254 20 Other steels prepared in accordance with the present invention have had a ductility, measured by percent elongation, as high as 50%. The greater the percent elongation, the better the drawability of the steel.

The term R value is a measure of the drawability of the steel and is a conventional term, defined more specifically in U .8. Patent No. 3,244,565. The larger the R value, the better the drawability of the steel. The term n value refers to the strain hardening coefficient of the steel, and is defined more specifically in Dieter, 6., Mechanical Metallurgy, p. 237, MeGraw-Hill, N.Y., N.Y., 1961. The higher the 11 value, the better the stretchabiilty of the steel.

The R value of the steel is the controlling factor where the deformation is pure draw, pure draw occurring when the metal undergoing deformation flows around a male die and into a female die. The n value of the steel is the controlling factor where the deformation is pure stretch, pure stretch occurring when the metal undergoing deformation is pulled over a male die. Most deformation operations called drawing are really combinations of pure drawing and pure stretching, so that both the R value and the n value are significant factors.

When the two steels listed in the above table were deep drawn into vegetable pans for refrigerators, the steel prepared in accordance with the present invention was successfully drawn without breakage, but there was significant breakage using the same steel without core killing.

The foregoing detailed description has been given for clearness of understanding only, and no unnecessary limitations should be understood therefrom, as modifications will be obvious to those skilled in the art.

What is claimed is:

1. A method for producing a deep drawing strip steel, said method comprising the steps of:

pouring into an ingot mold a molten steel consisting essentially of, in weight percent:

Carbon 0.06-0.12

Manganese 0.20-0.60 Phosphorous 0.03 Sulfur (maximum) 0.03 Copper (maximum) 0.05 Iron Balance allowing said molten steel initially to rim in said ingot mold;

then core-killing said steel, near the end of said pouring step, by injecting a deoxidizing element selected from the group consisting of aluminum, boron, vanadium, titanium and zirconium and combinations thereof downwardly into the interior of the stillmolten core of the ingot undergoing rimming and dispersing said deoxidizing agent in said molten core; said deoxidizing element being injected in a quantity sufficient to stop the evolution of rimming-causing allowing said ingot to solidify; rolling said ingot into strip; annealing said strip by heating it in a non-oxidizing atmosphere to an elevated temperature, of up to about 1300 F., said temperature being below the A temperature for said steel, to completely recrystallize said steel and provide it with a ferritic grain size of about 7-8 on the ASTM scale, with said grain size being substantially uniform throughout the entire strip, during said annealing the steel strip being in the temperature range of 1100 to 1300 F. for a period of about two hours; decarburizing said heated strip in a decarburizing atmosphere until the carbon content of the steel is no greater than 0.008 weight percent; said decarburizing step being initiated when the recrystallization has been completed; then immediately eliminating said decarburizing atmosphere and immediately cooling said heated strip without quenching, when the carbon content has been reduced to no greater than 0.008 weight percent, there being no substantial further anneal; and then temper rolling said strip. 2. A method as recited in claim 1 wherein said deoxidizing element is 0.02-0.08 weight percent aluminum. 3. A method as recited in claim 1 wherein said strip is decarburized, after said annealing step, at a temperature at least as high as said annealing temperature, up to about 1300" F.

4. A method as recited in claim 1 wherein: said decarburizing atmosphere is steam; the atmosphere around said strip is dried out when the carbon content of the strip has been reduced to 0.008 weight percent; and heating of the decarburized strip is stopped essentially immediately after said atmosphere has dried out. 1 5. A method as recited in claim 1 wherein: the atmosphere around said strip is rendered decarburizing when said grain size is about 7-8; and said grain size is about 7-8 at the conclusion of said decarburizing step, with the grain size remaining substantially uniform throughout the strip. 6. A method as recited in claim 1 wherein: the atmosphere around said strip is rendered nonoxidizing when the carbon content of the strip has been reduced to 0.008 weight percent; and heating of the decarburized strip is stopped essentially immediately after said atmosphere has been rendered non-oxidizing.

References Cited UNITED STATES PATENTS 3,178,318 4/1965 Shimizu et a1 148-3X 3,183,078 5/1965 Ohtake et 'al. -58X 3,239,390 3/1966 Matsukura et al. 148-12.l 3,281,286 10/1966 Shimizu et a1. 14812.l 3,333,987 8/1967 Schrader et al. l4812.1X 3,414,042 12/1968 Behrens et al 75-58X CHARLES N. LOVELL, Primary Examiner U .8. 'Cl. X.R. 14812.1, 16

@2 3 UNITED STATES PATENT OFFICE CERTIFICATE -OF CORRECTION Patent No. 6 Date'd January 19 19 71 Inventor) Melvin B. Gibbs, et a1.

It is certified that error appears in the above-identified patent and that said Letters Patent are hereby corrected as shown below:

at least as high as the said recrystallizing temperature but below the A temperature-.

Signed and sealed this 13th day of April 1971.

(SEAL) Attest:

EDWARD M.FLETCHER,JR. Attesting Officer WILLIAM E. SCHUYLER, JR. Commissioner of Patents 

